The present invention relates to an aromatic polyamide film having excellent adhesivity and productivity and to a method for producing the aromatic polyamide film. In more detail, the present invention relates to the aromatic polyamide film having excellent dimensional stability, or excellent strength and surface smoothness and to the method for producing the aromatic polyamide film.
Aromatic polyamide films have excellent strengths and excellent heat resistance. Therefore, the aromatic polyamide films have been noted in the uses of magnetic recording media, heat transfer recording media, dielectric materials for capacitors, and printed circuit boards. And the consumption quantity of the aromatic polyamide films has been expanded. The application of the aromatic polyamide films to the magnetic recording media is proposed in, for example, Japanese Unexamined Patent Publication 51-129201 (1976).
Then, the polar groups on the surface of an aromatic polyamide film, produced by a conventional technique, participate in intermolecular bonds at a high rate. And the reactivity of the polar groups is extremely poor. Further, when a functional layer is laminated to the film, the adhesivity of the film to the functional layer is inferior. When the aromatic polyamide film is used, for example, for magnetic recording media or printed circuit boards, the adhesivity of the aromatic polyamide film to an adhesive for fixing magnetic coatings and copper foils is poor. For the problem, the application of an easily adhesive coating or the like was proposed, but the effect was insufficient.
Further, the aromatic polyamide film is produced by a solution film production method. In the method, a film-producing dope comprising an aromatic polyamide solution is first cast on a support from a nozzle in a thin film state, and the film is formed by coagulating the aromatic polyamide with removing the solvent component of the film-producing dope from the thin filmy product cast on the support. But, the production cost of the aromatic polyamide film produced by the solution film production method is higher than that of a thermoplastic polymer film, such as a polyester film, produced by a melt film production method. The problem is mostly caused from needing a long time for extracting the solvent from the thin filmy product cast on the support and further from requiring a high cost for recovering the solvent.
Methods for extracting the solvent from the thin film-like aromatic polyamide solution cast on the support are largely classified into a dry method and a wet method. In the dry method among the methods, an aromatic polyamide solution is cast from a nozzle on a support such as a drum or an endless belt to form the thin filmy product. A solvent is then evaporated from said thin filmy product with hot air or the like, thus drying the thin filmy product until to have a self-supporting property. This method has a problem that a long time is needed for diffusing the solvent to the surface of the thin filmy product and further evaporating the solvent from said surface. For shortening the time, it is effective to enhance the temperature and speed of the hot air used for the drying. But the rapid thermal drying treatment causes the boiling of the solvent, and hence roughens the surface of the obtained film. It is difficult to apply the film having the rough surface to the use of magnetic recording media which require good surface flatness. There is further a problem that, when the solvent has a flammable, explosive or corrosive property, an infinite sum of installation cost is necessary for recovering the solvent and for avoiding the adverse effects of the solvent to environments.
On the other hand, the wet methods are classified into a method for directly extruding the aromatic polyamide solution from a nozzle into a coagulation bath in a thin film-like shape, and a method for casting the aromatic polyamide solution on a support similarly to the above-mentioned dry method and then guiding the cast solution together with the support into a coagulation bath. The method for directly guiding the aromatic polyamide solution into the coagulation bath has an advantage that the efficiency for extracting the solvent is high, because there is not a support. However, the form of the thin filmy product extruded from the nozzle as the planar product is remarkably easily deformed, and it is therefore extremely difficult to obtain a satisfactory film. While, the method for casting the aromatic polyamide solution on the support and then guiding the cast solution together with the support into the coagulation bath has a defect that the extraction of the solvent needs a long time, because the solvent can substantially not be extracted from the side on the support, and a defect that wrinkles are liable to be generated on the film due to the transverse-direction contraction of the film on the extraction. Consequently, the surface flatness of the film is deteriorated. In particular, it was difficult to obtain a flat surface which could be applied as the base of a magnetic recording medium.
Accordingly, the object of the present invention is to solve the problems of prior art to obtain in good productivity the aromatic polyamide film which utilizes the excellent heat resistance and high stiffness of the aromatic polyamide and simultaneously has easy adhesivity enabling the application of the polyamide film to printed circuit boards. Another object of the present invention is to obtain the aromatic polyamide film which can be applied to magnetic recording media and has easy adhesivity and good surface flatness. The objects are achieved by the present invention mentioned below.
The aromatic polyamide film of the present invention is the film comprising the aromatic polyamide, characterized by having microdomain structures having sizes of not less than 5 nm and not more than 600 nm. If the sizes of the microdomain structures are less than 5 nm, the adhesivity of the aromatic polyamide film to adhesives is remarkably deteriorated.
On the other hand, if the sizes exceed 600 nm, the breaking elongation of the film is deteriorated to make it difficult to handle the film in processing processes.
The method for producing the aromatic polyamide film in the present invention is a method for producing the aromatic polyamide film via a casting process of a film-producing dope, a coagulation process, a water-washing process, and a drying process. In the casting process of a film-producing dope, the film-producing dope comprising an aromatic polyamide solution is cast from a nozzle on a support in a thin film-like shape. In the subsequent coagulation process, a film is formed by coagulating the aromatic polyamide with at least adopting a wet bath system comprising the aqueous solution of a good solvent for the aromatic polyamide and with removing the solvent component of the film-producing dope from the thin filmy product cast on the support. In the water-washing process, the coagulated film is washed with water and simultaneously peeled from the support. Therein, the peeling process may be carried out at any stage during the water-washing process in response to the state of the produced film. And, in the subsequent drying process, the coagulated film is dried.
Further, in the method for producing the aromatic polyamide film of the present invention, the relation of a time T (minute) and the thickness t (mm) satisfies the following expression (1); (wherein, a time T means the time from the start of the coagulation process to the peeling of the coagulated film from the support and the thickness t means the thickness of the thin filmy product on the support just after cast). Thereby, the aromatic polyamide film which ensures flatness and in which the microdomain structures having proper sizes are formed can be obtained in good productivity.
1.2t (t+1)xe2x89xa6T xe2x80x83xe2x80x83(1) 
The sizes of the microdomain structures in the present invention are preferably not less than 10 nm and not more than 500 nm, further preferably not less than 20 nm and not more than 400 nm, especially preferably not less than 30 nm and not more than 300 nm.
The microdomain structures are estimated to be originated by the separation of phases in the process for coagulating the thin filmy product cast on the support, and enable the remarkable improvement of such adhesivity as mentioned later, because a multiplicity of extremely small voids are formed among the domains. The microdomain structures are formed over the whole film, and the sizes of the microdomain structures can be controlled by conditions for producing the film. Namely, the microdomain structures are formed in the process for extracting the solvent from the thin filmy product comprising the aromatic polyamide solution cast on the support, and the sizes of the microdomain structures can therefore be controlled by the extraction conditions.
Therein, the aromatic polyamide of the present invention is a polymer whose main chains comprise aromatic nuclei and amide bond groups as main constituents, wherein the main chain-forming substituents on the aromatic nuclei may mainly be oriented at meta-positions or at para-positions. Among the polymers, a polymer (para-oriented aromatic polyamide), in which para-oriented main chain-forming substituents on the aromatic nuclei (among which are forming the main chain of the polymer) are contained at 50 to 100 percent by mole of all the repeating units, is preferable. The rate is more preferably 60 to 100 percent by mole, further preferably 70 to 100 percent by mole, and furthermore preferably 80 to 100 percent by mole.
As the aromatic polyamide, a polyamide which contains repeating units represented by the following general formula (I) and/or general formula (II) in an amount of not less than 50 percent by mole based on the total repeating units is preferable. The rate is more preferably not less than 60 percent by mole, further preferably not less than 70 percent by mole. Other repeating units may be copolymerized or blended within the residual rate range. 
Wherein, Ar1, Ar2 and Ar3 include five groups represented by the following formulas. 
X and Y in the above-mentioned structural formulas are preferably selected from xe2x80x94Oxe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94Sxe2x80x94, and xe2x80x94C(CH3)2xe2x80x94. Further, the groups, in which some parts of hydrogen atoms on the aromatic rings in the above-mentioned structural formulas are substituted by substituents such as halogen groups (especially chlorine), nitro groups, C1 to C3 alkyl groups (especially methyl groups), C1 to C3 alkoxy groups, aryl groups, thioaryl groups, oxyaryl groups, and trialkylsilyl groups, are included. Further, the aromatic polyamides, in which hydrogen atoms in the amide bonds forming the polymer are substituted by other substituents, are also included.
From the aspect of characteristics, the polymer (para-oriented aromatic polyamide) in which the above-mentioned aromatic rings bound at the para-positions occupy not less than 50 percent by mole of the total aromatic rings is preferable. Thereby, the stiffness of the film is enhanced, and the heat resistance is also improved. The rate of the aromatic rings bound at the para-positions is more preferably not less than 60 percent by mole, furthermore preferably not less than 70 percent by mole.
In addition, it is preferable that the aromatic rings, in which hydrogen atoms on the rings are partially substituted by the above-mentioned substituents, occupy at a rate of not less than 30 percent by mole based on the total aromatic rings. Thereby, the moisture resistance of the film is improved, and the characteristics of the film, such as the change of the dimension and the deterioration of the stiffness, due to the absorption of moisture are improved. The rate is preferably not less than 50 percent by mole, more preferably not less than 70 percent by mole. Herein, more preferable substituents are a halogen group, an alkyl group, an alkoxyl group or a trialkylsilyl group.
Eleven aromatic nuclei represented by the following structural formulas can be exemplified as the aromatic nuclei having the para-orientations. 
The aromatic polyamide of the present invention can be produced by conventionally known methods. Therein, it is preferable that the inherent viscosity (a value measured at 30xc2x0 C. by using a solution of 0.5 g of the polymer solved in 100 ml of sulfuric acid at) of the polymer is preferably not less than 0.5, more preferably 1.0 to 8.0.
The aromatic polyamide of the present invention may be blended with an antioxidant, an antistatic agent, a mold release agent, one or more other additives and one or more other polymers in such an extent as not deteriorating the physical properties of the film.
The aromatic polyamide film of the present invention is preferably used as a composite material with a metal. Namely, it is used in the form that a magnetic metal layer, a metal element, a metal wiring or the like is formed on the film. Therein, the film is exposed to high temperature, when processed or used. When the obstruction of the functions of the composite material due to the generation of a halogen compound is worried, the ratio of halogen atoms to hydrogen atoms bound to aromatic rings in the repeating units of the aromatic polyamide is preferably controlled to no more than ⅓. The ratio is more preferably no more than ⅙, or it is especially preferable that the halogen is not contained.
The coefficient of thermal expansion of the aromatic polyamide film of the present invention is preferably 0 to 36 ppm/xc2x0 C. in the range of 30xc2x0 C. to 280xc2x0 C. When the film is laminated to a copper foil and then heated, the curvature of the laminate can thereby be reduced. The coefficient of thermal expansion is more preferably 1 to 30 ppm/xc2x0 C., further preferably 2 to 25 ppm/xc2x0 C.
The breaking elongation of the aromatic polyamide film of the present invention is preferably not less than 5% and less than 100%. It is preferable to satisfy the characteristic, because the suitability of the aromatic polyamide film for a processing process is improved. The breaking elongation is more preferably not less than 6% and less than 95%, further preferably not less than 7% and less than 90%, especially preferably not less than 8% and less than 85%.
The centerline average surface roughness Ra of the aromatic polyamide film of the present invention is preferably not less than 0.25 nm and not more than 25 nm. The value of the surface roughness Ra is more preferably not less than 0.3 nm, further preferably not less than 0.4 nm, especially preferably not less than 0.5 nm. In addition, the value of the surface roughness Ra is more preferably not more than 20 nm, furthermore preferably not more than 15 nm, especially preferably not more than 12 nm. When the surface roughness Ra is less than 0.25 nm, the handling of the aromatic polyamide film in the film-producing process and in the processing process is difficult, because the slip property of the aromatic polyamide film is not good, while it is also difficult to apply the aromatic polyamide film to the use of magnetic recording media due to the deterioration of electromagnetic transducing characteristics, when the surface roughness Ra is not less than 25 nm.
As a method for controlling the surface roughness of the aromatic polyamide film, a method for adding fine particles to the aromatic polyamide film, a method for coating the surface of the aromatic polyamide film, or the like may jointly be employed in addition to the method for controlling the sizes of the above-mentioned microdomain structures.
When the fine particles are added to the aromatic polyamide film, the following fine particles may be used. Inorganic fine particles such as the fine particles of metal compounds, for example, SiO2, TiO2, Sb2O3 and ZrO2, the fine particles of metals, and the fine particles of a compound represented by the general formula: M(OH)x or M2(CO)x (wherein, M is at least one metal element selected from the groups Ia and Ia in the periodic table, X is the number of 1 to 2). Organic fine particles such as silicone resin fine particles, polyimide fine particles, cross-linked copolymer fine particles, cross-linked polyester resin fine particles, acrylic resin fine particles or fluororesin fine particles (Teflon fine particles). Among the fine particles, the inorganic fine particles are more preferable than the organic fine particles from the viewpoint of heat resistance, and at least one kind of fine particles (especially SiO2) selected from SiO2, Sb2O3, and ZrO2 are especially preferable, because of having excellent dispersibility. Two or more kinds of the fine particles to be added may jointly be used.
The average particle diameter of the above-mentioned fine particles is preferably not less than 5 nm and not more than 600 nm, more preferably not less than 10 nm and not more than 500 nm, further preferably not less than 20 nm and not more than 400 nm, especially preferably not less than 30 nm and not more than 300 nm. The content of the fine particles is preferably not less than 0.001 percent by weight and less than 10 percent by weight, more preferably not less than 0.01 percent by weight and less than 1 percent by weight, further preferably not less than 0.03 percent by weight and less than 0.7 percent by weight, especially preferably not less than 0.05 percent by weight and less than 0.5 percent by weight.
The planar orientation coefficient of the aromatic polyamide film of the present invention is preferably not less than 0.16 and not more than 0.60. The planar orientation coefficient is more preferably not less than 0.20, further preferably not less than 0.25, especially not less than 0.30, in particular not less than 0.35, and not more than 0.58, further not more than 0.55. When the planar orientation coefficient is less than 0.16, the orientation of the aromatic polyamide film is insufficient, and the strength and Young""s modulus of the aromatic polyamide film are also therefore insufficient. Especially when a magnetic tape is produced from the aromatic polyamide film, the magnetic tape is insufficiently brought into a magnetic head, and sufficient electromagnetic transducing characteristics can thereby not be obtained. On the other hand, it is not preferable that the coefficient of plane orientation exceeds 0.60, because the frequency of the breakage of the aromatic polyamide film is remarkably increased when the aromatic polyamide film is produced, and further because the productivity of the aromatic polyamide film is drastically lowered.
The Young""s modulus of the aromatic polyamide film of the present invention in an arbitrary in-plane direction is preferably not less than 6,000 N/mm2 and less than 40,000 N/mm2, more preferably not less than 7,000 N/mm2 and not more than 30,000 N/mm2, further preferably not less than 8,000 N/mm2 and not more than 2,500 N/mm2, especially preferably not less than 9,000 N/mm2 and not more than 20,000 N/mm2. When the Young""s modulus is less than 6,000 N/mm2, the contact state of a magnetic tape produced from the aromatic polyamide film with a head is unstable, and the electromagnetic transducing characteristics of the magnetic tape is deteriorated. On the other hand, it is not preferable that the Young""s modulus exceeds 40,000 N/mm2, because the film producing property of the aromatic polyamide is deteriorated and hence because the productivity of the aromatic polyamide film is drastically lowered.
The thickness of the aromatic polyamide film of the present invention is preferably not less than 0.1 xcexcm and not more than 500 xcexcm, more preferably 0.3 to 350 xcexcm, further preferably 0.5 to 250 xcexcm, especially preferably 1 to 200 xcexcm.
The aromatic polyamide film of the present invention may be produced by an arbitrary method, if the method is a film-producing method capable of imparting the above-mentioned characteristics, the so-called solution type film-producing method. The solution type film-producing method includes a dry and wet method, a dry method, and a wet method, among which the dry and wet method and the wet method, especially the wet method, are preferable.
In the method for producing the aromatic polyamide film of the present film, the numerical value of a time T leading to the peeling of the coagulated film from the support is not less than 1.3 t(t+1), more preferably 1.4 t(t+1), especially preferably not less than 1.5 t(t+1).
In the casting process of the film-producing dope, the aromatic polyamide solution (the film-producing dope) may be a polyamide solution, obtained as it is by the production of the polyamide, but may be prepared by once isolating the polymer from said solution and then redissolving the isolated polymer in an organic solvent or an inorganic solvent such as sulfuric acid.
The method for producing the aromatic polyamide includes a low temperature solution polymerization method, an interfacial polymerization method, a method for reacting an isocyanate with a dicarboxylic acid, and a direct polycondensation method using a dehydration catalyst, among which the low temperature solution polymerization method is preferable, because the polymer having a high polymerization degree can easily be obtained. Namely, a method for polymerizing an acid chloride with a diamine in an aprotic organic polar solvent such as N-methylpyrrolidone (NMP), dimethylacetamide (DMAc) or dimethylformamide (DMF) is preferable. Therein, hydrogen chloride is by-produced on the reaction of the acid chloride with the diamine. When the hydrogen chloride is neutralized, an inorganic neutralizing agent such as calcium hydroxide, calcium carbonate or lithium carbonate, or an organic neutralizing agent such as ethyleneoxide, propyleneoxide, ammonia, triethylamine, triethanolamine or diethanolamine is preferably used. The obtained polymer solution may be used as the film-producing raw liquid (dope) as it is, but the polymer may once be isolated from the obtained polymer solution and then redissolved in the above-mentioned organic solvent or the inorganic solvent such as sulfuric acid to prepare the film-producing raw liquid (dope).
To the aromatic polyamide solution, if desired, an inorganic salt such as calcium chloride, magnesium chloride, lithium chloride or lithium nitrate may be added as a dissolution auxiliary. The aromatic polyamide solution as the film-producing dope has preferably a polymer concentration of 2 to 40 percent by weight, further preferably 3 to 30 percent by weight.
In addition, when the fine particles for controlling the surface roughness of the film are added to the aromatic polyamide solution, methods for the addition include a method for preliminarily adding the fine particles to a polymerization solvent or a dilution solvent and a method for directly adding the fine particles to the film-producing dope. The method for preliminarily adding the fine particles to the polymerization solvent is preferable, because the fine particles can be added and mixed under a low viscosity and further because the change of the viscosity can be reduced. It is also possible to use solid fine particles having a neutralizing ability for the neutralization reaction after the polymerization reaction, thus using the left fine particles as the fine particles for forming lumps. Further, a method for reprecipitating, washing and then gradually dissolving the once produced polymer in a solvent containing fine particles or a method for redissolving the reprecipitated polymer in a solvent and then mixing the obtained polymer solution with a solvent containing fine particles may also preferably be adopted.
The film-producing dope (aromatic polyamide solution) prepared as mentioned above is preferably cast on a support, when extruded from a nozzle, from the viewpoints of the flatness and surface smoothness of the film. A metal roll, a metal-made endless belt, a polymer film, a metal foil, or the like may be used as the support. The support may singly be used, or two or more of the supports may combinedly be used.
When the metal roll is used as the support, it is preferable to use the metal roll having a diameter of not less than 20 cm, more preferably not less than 30 cm, especially preferably not less than 40 cm, because it is difficult to obtain a sufficient contact time with the film, when the diameter of the roll is too small. When the metal-made endless belt is used, it is preferable to use the metal-made endless belt having a thickness of 0.3 to 3.0 mm, further preferably 0.4 to 2.5 mm, especially preferably 0.5 to 2.0 mm, from the viewpoint of handling. When the polymer film or the metal foil is used, the polymer film or the metal foil having a thickness of preferably not less than 0.5 xcexcm and less than 300 xcexcm, more preferably not less than 1 xcexcm and less than 250 xcexcm, further preferably not less than 2 xcexcm and less than 200 xcexcm, especially preferably not less than 3 xcexcm and less than 150 xcexcm, is used, and it is also preferable to use the polymer film or metal foil having the larger surface roughness of the back surface (which is not brought into contact with the film-producing dope) than that of the surface (which is brought into contact with the film-producing dope) to improve the handling property. Further, a mold release treatment using a silicone-based resin, a fluororesin, or the like, for imparting a mold release property may also be applied to the surface of the support.
It is preferable that the surface roughness Ra of the support is not less than 0.5 nm and not more than 50 nm, because the surface of the finally obtained film is maintained smooth. The surface roughness Ra is more preferably not less than 1 nm and not more than 45 nm, further preferably not less than 2 nm and not more than 40 nm, especially preferably not less than 4 nm and not more than 35 nm.
For the extraction of the solvent from the film-producing dope in the coagulation process, it is preferable from the point of extraction efficiency to use a bath (coagulation bath) using a good solvent for said aromatic polyamide. The bath for the extraction comprises an aqueous solution, especially the aqueous solution of the above-mentioned aprotic organic polar solvent. By a dry type method, the efficient of the extraction efficiency is bad, and it is difficult to produce the film having the microdomain structures.
In the coagulating process, a treatment, such as a treatment for drying the aromatic polyamide solution cast on the support with hot air for a very short time or a treatment for blowing air having a constant temperature and a constant humidity on the cast aromatic polyamide solution to achieve the optical isotropy of the liquid crystal solution, may, if necessary, be applied to the aromatic polyamide solution cast on the support, before guided into the wet type bath. The time for the treatment is also contained in the time T (minute) from the start of the coagulation process to the peeling of the coagulated film from the support in the present invention.
In the production method of the present invention, it is preferable to orient the coagulated film in at least one direction to give an area stretch ratio of 1.01 to 15 during the process after the finish of the coagulating process. Thereby, the aromatic polyamide film which ensures good flatness and simultaneously in which the microdomain structures having proper sizes have been formed can also be obtained in good productivity. The area stretch ratio is preferably 1.05 to 13, further preferably 1.1 to 11, especially preferably 1.2 to 10. The orientation temperature in a wet state including the state in a water bath is preferably room temperature to 95xc2x0 C. Or, the orientation temperature in a dry state including the state in the drying process is preferably 100 to 600xc2x0 C.
After the finish of the coagulating process, the coagulated film, namely the aromatic polyamide film is guided to the water-washing process and then subjected to a solvent-removing treatment and a desalting treatment in the wet bath. During the water-washing process, the aromatic polyamide film is peeled from the support. The peeling work may be carried out at any stage during the water-washing process in response to the state of the produced film. Namely, the peeling work may be carried out before, during or after the water-washing work.
In the drying process after the water-washing process, the drying temperature is preferably 50 to 250xc2x0 C., further preferably 70 to 240xc2x0 C., especially preferably 80 to 220xc2x0 C. The drying time is preferably 1 second to 30 minutes, further preferably 3 seconds to 20 minutes, especially preferably 5 seconds to 15 minutes. At the time, one or more heating rolls or an oven may be used.
In addition, after the drying process, it is preferable to thermally treat the coagulated film at a temperature of not less than 250xc2x0 C. and not more than 700xc2x0 C. By the thermal treatment, the effect of the present invention becomes more remarkable. The thermal treatment temperature is preferably 260 to 650xc2x0 C., further preferably 270 to 600xc2x0 C., especially 280 to 550xc2x0 C. The thermal treatment time is preferably 1 second to 60 minutes, further preferably 5 seconds to 30 minutes, especially preferably 10 seconds to 20 minutes.
As the oven used for the drying process or the subsequent thermal treatment, a clip-tenter, a pin-tenter or the like may preferably be used.
The method for orienting the coagulated film, at the stage that the coagulated film is guided to the heating roll or the oven, includes a usual transverse orientation method using the clip-tenter, a simultaneous biaxial orientation method using a simultaneous biaxial orientation machine, and a machine direction (longitudinal) orientation method comprising heating a water-removed film with an infrared light heater, an induction heating roll or the like and orienting the film by the utilization of a difference between the circumferential speeds of a low speed roll and of a high speed roll. The methods may suitably be combined with each other. If necessary, the biaxially oriented film may again be oriented in the machine direction, in the transverse direction or in both the machine and transverse directions, or be treated with relaxation while heated.